Calorie

A calorie is a unit of energy.
Basically it tells you how much Joules of energy you can produce.
EDIT:
Psychics: Amount of energy required to raise of 1 gram of H2O by 1 Degree (Celsius)

It's a measurement of energy...

It's also approx. 4 Kilojoules.

Calorie is a unit of energy equal to 4 Kilojoules, did you ever take Chemistry / Science ?

Also, the calories that we see on food 'n shit is actually in kilocalories. Those aren't "true" calories.